Switching to Digital Television Broadcasting

We are now faced with a transition to digital broadcasting. But what does it actually mean? Digital technology was already present in radio and TV production twenty years ago. Now it is time to implement it in transmitters, one of the final links toward listeners and viewers who depend on terrestrial reception. Digital television is a broad term. We should distinguish between digital production (making of the program) and actual broadcasting - transmitting signals to our rooftop antennas. It is possible to produce TV program with the latest digital technology and transmit it in analog or to produce in the old fashioned analog way and transmit it in digital. So be careful when you talk about digital television.

If you don't know what is the difference between analog and digital signals, take a look at this simple example. You can tell the size of your TV screen in two different ways. If you show the size with your hands then this is an analog value. The size of screen is represented with the distance between your left and right hand. This distance can be arbitrary small and the value (in this case the distance between hands or screen size) can be any value between zero (both hands together) and the maximum span of your hands.

To tell the screen size in digital you would measure it with meter tape or ruler by noting the number at the end of screen and rounding that number to some appropriate number of decimals. This number now represents the digital (numerical) value of the screen size. What you have done is actually an analog to digital conversion. Now you have a number that represents the size of the screen. Since you have rounded the number it is not an exact value of the screen size but a value that is sufficient for most purposes. To measure exact value it would take an infinite number of decimal places which is impossible to achieve and usually we don't need that exact number.

Why are we switching to digital television broadcasting? The main reason is more effective use of the radio-frequency spectrum. In analog world each TV channel (program) occupied one frequency (radiofrequency channel). The bandwidth of this channel is between 6 MHz (USA) and 8 MHz (Europe). The amount of radio-frequency spectrum available to television broadcasting is limited. Therefore there is a fixed number of channels (frequencies) that can be used for television broadcasting. We can reuse these frequencies if we can ensure that transmitters transmitting on the same frequency are far enough to avoid interference. Many countries have used all frequencies assigned to them and further expansion of television stations with terrestrial broadcasting was not possible. A new technology had to be developed. And it was. Audio and video signals are digital and can be effectively compressed with various compression methods. Digital broadcasting can use one frequency channel to broadcast a package of compressed television, radio and data services called multiplex.

Different digital standards have been developed. ATSC is used in North America; DMB-T is used in China; ISDB-T is used in Japan while Europe, Australia and many other countries have decided to use DVB-T. Each of these standards transmits a stream of digital data. This transport stream contains compressed audio and video. There are two popular codecs or compression algorithms (standards) used for compression: MPEG-2 and the newer, better standard MPEG-4. All these standards are incompatible and you need a digital receiver (set-top-box) which is compatible with transmission and compression standards used in your country.

What brings us digital television broadcasting? More channels, better picture and sound quality, possibility for high definition television, wide screen picture, multi-channel sound and new services. But don't buy a new plasma or LCD TV if you already have a working TV set at home. Get a digital set-top-box and enjoy new services with minimal cost.

If you are interested in technical details of digital terrestrial television you can take a look at the DVB-T transport stream analysis and status of the DVB-T in Slovenia, Austria, Italy, Australia, Latvia, Estonia, and some other countries.